1. Field of the Invention
The present invention relates to methods and devices for the detection of underground anomalies, and more particularly to the use of an electromagnetic gradiometer mounted to a vehicle trailer to detect linear underground anomalies crossing under border patrol roadways.
2. Description of Related Art
In the general case of having to detect linear underground anomalies from the surface, the orientation of the linear underground anomaly, if it exists, will be unknown. And its exact area will also be unknown unless there is some related surface feature or objective the anomalies involve.
However, in the case of detecting linear underground anomalies crossing under border patrol roadways, if the roadways are tight against a border, the linear underground anomalies will more or less pass orthogonally underneath. The search area required thus reduces from a two-dimensional field to a one dimensional line, the track of the roadway.
Various kinds of conventional technologies have been employed to detect and location underground anomalies, mines, and other structures. Many have used earth penetrating radar techniques. Others look for the secondary emissions from buried objects that occur when they are illuminated by primary radio sources. It is also fairly well understood that some radio frequencies will propagate through the ground better than others, and that will depend on soil conditions.
Primary electro-magnetic (EM) waves will interact with underground objects and infrastructures to create scattered EM-waves that are detectable on or above the earth's surface with a gradiometer. The Stolar, Inc. (Raton, N. Mex.) DeltaEM-gradiometer survey system provides a tool that can generate subsurface geophysical imaging capabilities with greater sensitivity, range (distance), and flexibility over existing instrumentation. In efforts using local radio sources, EM gradiometry has been shown to be a promising technique. The synchronized EM-gradiometer instrumentation is a narrow-band receiver that can discriminate against the spectra noise components and operate in the low ionosphere-earth waveguide noise band, thus maximizing the detection threshold sensitivity of the instrumentation.
EM-gradiometers capitalize on their high threshold detection sensitivities to scattered EM-waves in the ELF/VLF bands, 3-3000 Hz and 3-30 kilohertz. Synchronization to the primary wave in the ELF/VLF bands enables very narrow-band detection with threshold detection sensitivity in the picoTesla (pT) range. Theoretical investigations have found that the secondary EM fields are 20-60 dB below that of the primary EM field components. A significant instrument design issue is the detection of the secondary fields in the presence of the much larger primary field components. This has been solved by the careful design of the gradiometer antennas that achieves 70 dB of primary field suppression.
Two important advantages in underground anomaly detection have been achieved. First, the magnitude of the scattered secondary wave from them increases as frequency decreases. Thus, waves in the ELF/VLF bands have a significant advantage in detection. Second, the attenuation rate of EM-waves in the ELF/VLF bands through soil/rock is very low, so deeply buried structures can be illuminated and detected. The structures may be empty passageways or may contain electrical conductors serving some utility and ventilation needs.